This disclosure generally relates to treatment of oil and gas reservoirs.
Hydrocarbons (oil, condensate and gas) are typically produced from wells that are drilled into the formations containing them. For a variety of reasons, such as inherently low permeability of the reservoirs or damage to the formation caused by drilling and completion of the well, the flow of hydrocarbons into the well may be low. In this case, the well can be stimulated, using a variety of techniques, including hydraulic fracturing, chemical stimulation (sometimes referred to as acidizing or matrix acidizing), or a combination of the two (referred to as acid fracturing). During stimulation of a subterranean formation, a treatment designed to treat an area of a formation at or near a wellbore, otherwise known as a matrix treatment, may result in particular challenges.
In matrix acidizing of carbonates, an acid often used for stimulation of the carbonate formations is hydrochloric acid (HCl). A goal in matrix acidizing is for an acid to create wormholes in the formation, thereby stimulating the formation. However, the reaction kinetics with carbonate and the corrosion rates of parts of the formation, including those in contact with the wellbore, will increase with temperature. As a result, the corrosion rates at high temperatures are difficult to control and the reaction kinetics can result in an inefficient stimulation.
In acid fracturing, in addition to the challenge of controlling corrosion rates, the rate at which the acid may spend when injected into a hydraulic fracture, and the distance of the acid from the wellbore, may each be affected by the reaction kinetics of the acid and the width of the hydraulic fracture.
The industry may use an emulsified acid system as a retarded acid system to aid in stimulation of a subterranean formation, such as a carbonate reservoir, to slow and control reaction and corrosion rates. The emulsified acid system may also generate some control of fluid loss, which can allow for more fluid to remain in a fracture and thus a wider hydraulic fracture.
Emulsions generally comprise two immiscible phases. The immiscible phases may include a continuous phase and a discontinuous phase. Emulsions may be used in various oil and gas applications. For instance, emulsions may be used for subterranean treatment applications, including drilling, production and completion operations.
Emulsion stabilizing agents may be surfactant-based. In this case, the emulsion stabilizing agents generally interact in such a way where the surface tension of the interface between water and oil is decreased, which may slow a natural tendency of the immiscible phases to separate.
One industry formulation of an emulsified acid system, often used in matrix and acid fracturing treatments, is an oil-outside-phase formulation with 30% oil and 70% acid. The acid used is most commonly HCl. However, these systems are often subject to high friction pressures and relatively low viscosities at some temperatures, resulting in small hydraulic fracture width and faster spending of the acid being used in the treatment. The viscosity of the emulsion generally decreases with increasing temperature, and often becomes minimal above 100° C. Such limitations may reduce the length of the conductivity fracture and ultimately inhibit productivity.